Observations of element abundances in the diffuse interstellar medium
and in molecular clouds are subject to depletion and fractionation
effects
(Spitzer & Jenkins
1975,
D.G. York, in preparation), but some
evidence on total abundances comes from X-ray absorption measurements
(Ryter, Cesarsky &
Audouze 1975),
which are compatible with (but do
not necessarily demand) an abundance gradient reaching an enhancement
by a factor of 3 or so at the galactic center. Infrared observations
of Sgr A(W) suggest an enhancement of this order in Ne and presumably
other heavy elements relative to H
(Aitken, Griffiths &
Jones 1976).
Molecular observations imply that the isotopic ratios
13C/12C,
14N/15N, 17O/16O (which are
expected to be enhanced by the CNO cycle)
increase by small but significant amounts in the order: Solar System,
galactic disk
(R
± 4 kpc where
R is the
solar galactocentric
radius), galactic center by factors of up to 3 or 4, but without any
significant gradient in the galactic disk
(Penzias 1980,
Wannier 1980).

If, as has been rather naturally supposed in some chemical evolution
models (e.g.
Talbot & Arnett 1974,
Vigroux, Audouze &
Lequeux 1976)
13C, 14N and 17O are secondary
nucleosynthesis products from 12C and
16O initially present in the progenitor star, one expects their
abundances relative to primary products like 12C and
16O to vary (in
first approximation) as the total abundance of primary elements
relative to hydrogen, assuming that the efficiency of secondary
production is independent of stellar composition. (This assumption
actually seems rather doubtful: cf.
Sweigart & Mengel
1979,
Rocca-Volmerange &
Audouze 1979.)
The galactic center isotopic
abundances conform to this trend (N/O is unknown there), but the
isotope and N/O ratios in the disk do not. Taking into account the N/O
ratio in other galaxies (see below), several authors (e.g.
Smith 1975,
Edmunds & Pagel 1978,
Alloin et al. 1979)
have proposed that a
substantial component of nitrogen is a primary product whose yield
(relative to oxygen) depends on the age and/or initial mass function
of the underlying stellar population and is fairly constant in any one
galaxy, but
Wannier (1980)
argues that a more radical revision of
ideas on galactic chemical evolution is needed. We believe that
classical secondary production may well be taking place near the
centers of our own and some other galaxies (like M81 where evidence
for a large N abundance is good; M. Peimbert, in preparation), but
that in most other places the evidence favors either a primary process
- meaning that 13C, 14N, and 17O come
from the CNO cycle in stars that
are already self-enriched in carbon by helium burning and mixing - or
a form of secondary processing in which the shortage of "seeds" in
low-abundance stars is more or less compensated by an increase in the
vigor of mixing processes. (This could also have some relevance to the
s-process abundances discussed in
Section 2.4.) The many anomalies
encountered in globular cluster red giants (e.g.
Kraft 1979)
suggest that both of these effects may well be operating.

Several other isotope ratios
(Penzias 1980,
Wannier 1980)
are also of interest. D/H has an abundance gradient decreasing inwards,
which agrees with cosmological synthesis and a limited degree of destruction
by astration; however, the nonzero value at the galactic center
implies a modest inflow of unprocessed material or a separate
production source
(Audouze 1977).
18O/16O is enhanced at the
galactic center while 15N/14N is depleted, both by
about a factor 2. Sulphur and silicon isotopes are effectively unchanged.